A standard magnetic grab such as described in French patent document 2,441,577, German patent document 2,704,118 and European patent application 90,746 normally has two groups of permanent magnets each having a lower surface sitting atop a respective pole piece and an upper surface bridged by a piece of mild steel, forming in effect a downwardly pointing U-magnet. The magnets in one leg are polarized oppositely to those in the other leg so that one of the pole pieces is the north pole and the other the south pole of the magnet. A rotor situated between the legs of this symmetrical magnet can be rotated to juxtapose its north pole with the north-pole leg and its south pole with the south-pole leg in an active position to add together the magnetic fields and to make the grab capable of lifting a magnetically attractable object engaged with the pole pieces. When reversed, with the rotor north pole turned toward the south-pole leg and the rotor south pole turned toward the north-pole leg, all flux flows between the two legs through the rotor and the grab has no appreciable lifting capacity.
Such an arrangement is fairly tall so that it has a high center of gravity and therefore can be fairly difficult to use. In addition it has a fairly small footprint so that lifting a large object with it requires that it be carefully centered on the object.
It has been suggested in copending application 07/816,736 filed Jan. 3, 1992 to provide a magnetic grab with an even number of main permanent magnets in an annular array with each magnet horizontally polarized and having a north pole and a south pole with each pole spacedly angularly juxtaposed with the pole of the same polarity of the adjacent magnet. Respective magnetically permeable pole pieces are angularly interleaved with the magnets so that the pieces between north poles are polarized north and the pieces between south poles are polarized south. A magnetically permeable horizontal plate is spaced above the pole pieces and a nonmagnetic disk is rotatable about the axis immediately above the magnets and pieces and below the plate and has angularly spaced zones in the same even number as the pole pieces. A switching permanent magnet on the disk in each zone is vertically polarized with the magnet of each zone polarized vertically oppositely to the switching magnets of the angularly adjacent zones. The disk is movable between an active position with the north poles of a first half of the switching magnets turned toward the north-polarized pole pieces and the south poles of a second half of the switching magnets turned toward the south-polarized pole pieces and an inactive position with the poles oppositely oriented. Thus in the active position the fields of the main magnets are depressed by those of the switching magnets below the pole pieces and in the inactive position the magnetic flux of the main magnets is shunted mainly through the plate.
Such an arrangement is a substantial improvement, but is still fairly complex.